The present invention relates to an exhaust outlet conduit intended for internal combustion engines and constructed to function as a resonator for the exhaust waves exiting from the cylinder or cylinders to which it is connected.
The resonance phenomenon in exhaust outlet conduits of an internal combustion engine has a significant effect on engine power, engine efficiency and fuel consumption. Consequently, it is normal practice in present times to select conduit diameters and conduit lengths which enable said conduits to function as quarter-wave resonators at a given engine speed, determined by the type of engine concerned and the use for which it is intended. The engine speed at which resonance is desired differs considerably between a car engine for normal use and an engine used in high-speed motor cycle competitions.
By utilizing the resonant phenomenon in the exhaust outlet conduit, which gives rise to a standing wave in the conduit, it is possible to improve considerably the filling of the cylinder of, e.g., a four-stroke engine, since it is possible in this way to engender an underpressure in the exhaust port at that time when both the suction valve and the exhaust valve are open, thereby assisting in the removal by suction of residual exhaust gases and the introduction of fresh gases through the suction port before the exhaust valve closes.
The same conditions prevail in principle in two-stroke engines, although such engines are more sensitive and normally require a counter-pressure wave to be fed back through the exhaust conduit, in order to prevent the fuel mixture from escaping through the exhaust port at the end of the overlapping exhaust and overflow phases.
The afore described effect capable of being achieved with standing waves in the exhaust outlet conduit has a great significance on engine performance. For example, when engendering an underpressure in the vicinity of the exhaust port at the end of the exhaust stroke, in accordance with the aforegoing, the residual, combustion gases which are not ejected by the piston and thus remain in the cylinder are sucked out from the cylinder and, furthermore, as a result of the aforesaid overlap between the times at which the exhaust valve and suction valve are open, the cylinder will be filled with a fresh fuel mixture prior to the occurrence of suction generated by downward movement of the piston.
This technique is employed, for instance, in speedway motor cycles, in which the exhaust outlet conduit is given a funnel shape in order to improve its evacuating effect. This enables a relatively high power output to be obtained at maximum engine speeds. The sound level of the engine, however, is very high and at times unacceptably so.
In addition to minimizing the amount of fresh fuel mixture that is lost through the exhaust outlet port, it is also possible when utilizing the aforesaid counterpressure wave in two-stroke engines to feed back the fuel mixture pressed out from the cylinder therewith achieving further supercharging of the cylinder.
Thus, there are utilized the natural resonance oscillations of the air column present in the exhaust outlet conduit, which therewith functions as a quarter-wave resonator. One serious drawback with this arrangement, however, is that resonance is only reached at a specific engine speed and odd multiples thereof. Consequently, it is possible to achieve optimum engine performance only at one single engine speed, the resonance effect being greatly impaired immediately the engine is driven at a different speed to this single speed. Resonators can also give rise to a countereffect outside the resonance range.
Consequently, in the case of sound damping constructions it has been proposed to use exhaust outlet conduits provided with slot-like openings in the conduit wall, see for instance FR No. 536 812, DE No. 26 41 01 and DE No. 21 46 842. These slots normally have a widest part which faces the cylinder, although the reverse may also be the case.
The present invention is based on the concept that the speed range of an internal combustion engine over which the exhaust outlet conduit can function as a power-enhancing resonator can be increased considerably by incorporating a slot or a row of holes in the conduit, provided that the slot or the row of holes is so configured that the total opening area increases along the conduit in a direction away from the cylinder.
When provided with one or more openings having an increasing area in accordance with the above, a conduit which functions as a quarter-wave resonator at a given lowest fundamental frequency will also function as a resonator at higher frequencies, since the length of the resonant part of the conduit decreases such that, seen from an oscillating aspect, the end of the conduit will appear to move, in an illusory fashion, along the opening or openings in a direction towards the cylinder at increasing frequency.
In this way, the power-increasing effect of the resonance phenomenon in the exhaust outlet conduit can be utilized over an extensive range of engine speed, which is highly beneficial both from the aspect of economy and of performance.
The drawbacks with the above described construction are that the evacuation performance of this construction is lower than that of a funnel-shaped conduit and that the same distinctive resonance effect obtained at maximum engine speed when using a funnel-shaped conduit cannot be obtained with the aforedescribed embodiment.